Messier 3...

Object: Messier 3 (NGC 5272)
Type: Globular cluster
Constellation: Coma Bereneces
Distance: 34,000 light years

Date: April 25th 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 20 x 150s (2x2 binned) each for RGB, flats for each channel, bias as darks (hot pixel removal in Astroart).

Globular clusters are some of the oldest objects in the universe. They consist of vast, spherical agglomerations of stars, typically hundreds of thousands in number, and exist in galactic halos in orbit around larger galaxies.  The Milky Way galaxy has around 150 globular clusters in attendance, with more massive galaxies having proportionally larger numbers.

M3 was discovered on May 3, 1764, and was the first object in the Messier catalogue to be discovered by Charles Messier himself. Messier originally mistook the object for a “nebula without stars”. With a larger and better quality telescope, the object was resolved into stars by William Herschel around 1784, who first coined the term “globular cluster”.

Stellarium map showing location of image field

From the UK, M3 can be found high above the southern horizon at around midnight in late April, between the bright stars Arcturus and Cor Caroli. The cluster has a bright core with a diameter of about 6 arcminutes and spans a total of 12 arcminutes (or just under the half the diameter of a full moon). This cluster is one of the largest and brightest of its type, and is made up of around 500,000 stars. It is estimated to be 11.4 billion years old.

I had originally planned to gather both luminance and colour data for this object, but the seeing was very poor and I struggled to get the auto-guiding to run below 1.3” RMS, so I restricted myself to collecting 2x2 binned sub-frames, where the guiding requirements aren’t as demanding.

However, I was quite pleased with the way the RGB image scrubbing up without any additional luminance data, so I’ve decided to let well enough be. 

If you look closely, the image shows up the strange straight lines of stars that appear to run from the edges of the cluster, which Stephen O'Meara illustrated for M3 in his book "Deep-Sky Companions - The Messier Objects".  It also shows up the 16th magnitude spiral galaxy NGC 5263, visible at the centre right of the main image as small fuzzy dash of light and lying some 196 million light years away...

Messier 5 - Globular cluster in Serpens...

Object: M5 (NGC 5904)

Type: Globular cluster
Constellation: Serpens
Distance: 2450 light years

Date: 4th & 5th May 2018
Equipment: SXV-H9, Vixen 114mm f5.3 ED refractor, NEQ6 mount, guiding with Lodestar X2/PHD

Subframes: 50 x 30s, 30 x 100s and 20 x 300s luminance, 25 x 100s each for RGB (2 x 2 binned), 20 flats for each channel.

At a magnitude of 6.4, M5 can readily be glimpsed through binoculars as a "fuzzy star", midway between fourth magnitude stars Epsilon Serpentis and 109 Virginis.

M5 often seems to be overlooked in favour of its flashier cousin, the "Great Globular Cluster" M13 in nearby Hercules, yet it is only half a magnitude fainter and in many ways is a more rewarding object to view through a telescope.  In my 8-inch Vixen VC200L , M5 shows up as a cluster of pin sharp stars, gradually brightening, yet still resolvable, right the way to its bright core.  M13, although bigger and brighter, doesn't offer as "granular" an appearance, at least not to me.

For imaging, the challenge is to capture M5's outlying stars, without having a "burning out" look to its bright core.  Luminance stacks of three different exposure lengths were layered and blended in PaintShop, to give a final luminance frame showing an apparent resolved core of stars.  This was combined with an RGB stack in AstroArt to give the final image.

Skies were very clear for the two nights needed to acquire the frames I wanted, but the seeing seemed a little variable, giving rise to a PHD guiding graph that often resembled an ECG trace.  Still, PHD worked its magic and the stars still came out OK-ish, with only a trace of distortion towards the outermost corners of the final image.  Clear skies also helped keep the background noise down, without having to resort to clipping or smoothing.

And it was so nice be able to sit out at night in the observatory at a balmy 10 degrees...

More info on this galactic shepherd can be found here...